The present invention relates to electret condenser microphones for use in various audio equipments, telephones, etc.
Electret condenser microphones are of two basic types: (1) wheel electret, having a vibratory diaphragm formed of an electret; and (2) back electret, having an electret fixed by fusing, for example, to a fixed electrode.
One such prior wheel electret condenser microphone will be described below with reference to FIG. 16 of the accompanying drawings. The wheel electret condenser microphone includes a cup-shaped metallic case 1 having a top end wall 1a with a plurality of sound-receiving apertures 1b. A metal ring 2 is secured to an inner surface of the end wall 1a and a stretched vibratory diaphragm 3 is fixed to an end face of the metal ring 2. The vibratory diaphragm 3 has a deposited metal film 3a on its one surface facing the end face of the metal ring 2. The vibratory diaphragm 3 is separated by a spacer ring 4 from a fixed back plate or counter electrode 5 so that the diaphragm 3 and the counter electrode 5 form a condenser 6 whose capacity varies with the vibrations of the diaphragm. The counter electrode 5 has pressure-equalizing holes 5a communicating with a rear cavity 7 behind the counter electrode 5 to equalize static pressure across the diaphragm 3. The rear cavity 7 is defined by a cup-shaped insulating support 8, which fixedly supports on its open end the counter electrode 5 and electrically separates the counter electrode 5 from the metallic case 1 and the diaphragm 3. A field-effect transistor (FET) 9 used for impedance conversion is disposed in the rear cavity 7 and electrically connected with the counter electrode 5 via an input lead 9a. The input lead 9a has one end which is either held in contact with the counter electrode 5 or fixed by spot welding to the counter electrode 5. The field-effect transistor 9 has an output lead 9b connected by soldering to a printed-circuit board 10 on which the insulating support 8 is mounted. The printed-circuit board 10 carrying thereon the insulating support 8, the field-effect transistor 9 and the counter electrode 5 is assembled with the cup-shaped metallic case 1 by clinching an open end edge 1c of the metallic case 1 over and around the periphery of the printed-circuit board 10, with all the components 5, 8, 9 received in the metallic case 1.
FIG. 17 is an exploded view of the prior electret condenser microphone shown in FIG. 16, with parts shown upside down for a purpose of illustration of the manner in which the microphone is assembled. The fixed counter electrode 5, the insulating support 8, the field-effect transistor 9 (not shown in this figure as it is mounted in the insulating support 8), and the printed-circuit board 10 are assembled together into a preassembled built-in amplifier block. In assembly, the metal ring 2 carrying on its upper end face the diaphragm 3, the spacer ring 4, and the amplifier block having the counter electrode 5 facing downward are placed in the cup-shaped metallic case 1 successively in the order named. Then an open end edge of the metallic case 1 is bent into an inwardly curled edge 1c (FIG. 16) firmly clinched over and around the periphery of the printed-circuit board 10. The electret condenser microphone is thus assembled.
With this construction, when the diaphragm 3 of the electret condenser microphone is vibrated by acoustic pressures impinging thereon through the sound-receiving apertures 1b, the diaphragm 3 produces a capacitance change between the diaphragm 3 and the fixed counter electrode 5 of the condenser 6. Since the electrical impedance of the condenser microphone is relatively very high at audio frequencies, a direct current electric field is applied in which instance the field-effect transistor 9 is used as an impedance converter.
As described above, the fixed counter electrode 5 is structurally separated from the metallic case 1 and in order to form a condenser 6 by and between the fixed counter electrode 5 and the vibratory diaphragm 3, the spacer ring 4 must be disposed between the counter electrode 5 and the diaphragm 3. The prior electret condenser microphone thus constructed has a relatively large number of component parts, is complicated in construction, requires a time-consuming assembly and is costly to manufacture. Furthermore, with this large number of components retained in the case 1, there is provided only a small room available for the formation of the rear cavity 7.